1. Field of the Invention
The present invention relates to a lithographic apparatus, an article support member, and a method for manufacturing a device. More specifically, one embodiment of the invention relates to a lithographic apparatus having an illumination system for providing a projection beam of radiation; an article support member for supporting an article to be placed in a beam path of the projection beam of radiation on the article support; and a clamp for providing a clamping pressure for clamping the article against the article support during projection.
2. Description of the Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that circumstance, a patterning device or patterning structure, such as a mask, may be used to generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. comprising part of, one or several dies) on a substrate (e.g. a silicon wafer) that has a layer of radiation-sensitive material (resist). In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through the projection beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction.
Designing article supports such as wafer tables or reticle supports in a lithographic system is a necessary process. Conventionally, the article support is provided with protrusions that are arranged to improve the flatness of the substrate. European patent application EP0947884 describes a lithographic apparatus having a substrate holder wherein protrusions are arranged to improve the flatness of the substrate in a vacuum clamped wafer support table. These protrusions have a general diameter of 0.5 mm and are located generally at a distance of 3 mm away from each other and thereby form a bed of supporting members that support the substrate. For an electrostatic clamp, typically, the height of the protrusions lies in the range 1 μm–15 μm. For a vacuum clamp, these protrusions generally have a height of 100 μm. Due to the relative large spaces in between the protrusions, contaminations possibly present generally do not form an obstruction for the flatness of the substrate, since these will be lying in between the protrusions and will not lift the substrate locally.
In the context of this application, the said “article” may be any of the above mentioned terms wafer, reticle, mask, or substrate, more specifically terms such as a substrate to be processed in manufacturing devices employing lithographic projection techniques; or a lithographic projection mask or mask blank in a lithographic projection apparatus, a mask handling apparatus such as mask inspection or cleaning apparatus, or a mask manufacturing apparatus or any other article or optical element that is clamped in the light path of the radiation system.
From the above identified patent application it is known that the design of these protrusions requires careful choice of design parameters that are influential to the pressure applied to the wafer, and thus its vertical position and flatness. Other publications such as EP1241706 are related to an electrostatic wafer support table, wherein a substrate is electrostatically clamped to the wafer support table. Known electrostatic clamps are usually provided with an arrangement arranged to use a backfill gas, which is used to fill the spaces formed between the protrusions, a base plate that forms a ground layer for supporting the protrusions and the backside of a substrate clamped by the substrate holder. Such a backfill gas enhances the heat transfer capability from the substrate to the substrate holder. In order not to spill this backfill a wall may be present having a contour that generally follows the contour of the substrate and which provides, when the substrate contacts the substrate holder, a confining environment is provided that restricts gas-outflow. It was found that, also for electrostatic clamping configurations, especially for electrostatic clamping configurations where a hard rim is present for confining a backfill gas to the back of a substrate to be irradiated, the wafer tends to be supported unevenly, especially near the boundary of the protrusion configuration, due to the termination of support near the boundary edge. This may cause, depending on the degree of overhang, an upward lifting of the wafer or a downward “sagging” near the edges, which may result in unacceptable imaging quality. Hence, here, also the design parameters can change substantially the amount of levelness of an article to be supported by the article support.
However, design choices are limited by manufacturing tolerances. It has been found that manufacturing tolerances can be a problem to meet the specifications of particular article support designs, in particular relating to vacuum seal position, burl position and stiffness, and di-electricum thicknesses.